Rubber is conventionally compounded (mixed) with various ingredients in order to prepare rubber products. Such ingredients typically include curatives, cure activators, resins, reinforcement pigments, processing aids as well as other pigments.
While carbon black is a conventionally preferred reinforcement for many rubber products, silica is often used, often in conjunction with carbon black, as a particulate filler. The silica may have a reinforcing effect, depending on its particle size and whether it is used in combination with a suitable coupling compound.
Siliceous pigments (silica) are often used in various rubber compositions in the form of a precipitated, hydrated silica, typically of the type obtained from the precipitation of a soluble silicate. For example, reference may be made to U.S. Pat. No. 2,940,830 for a method of preparing precipitated silica.
Conventionally, the silica (siliceous pigment) has a silicon dioxide content of more than 50 percent, and more typically at least 80 percent by weight.
While it is acknowledged that there are various forms of colloidal finely-divided silicas which may be used in rubber compositions such as, for example, arc, pyrogenic and precipitated silicas, the precipitated silicas are usually preferred.
Good and homogeneous dispersions of silica pigment in rubber is often and usually difficult to obtain with conventional industrial mixing procedures with relatively short mixing times, and particularly where relatively large amounts of silica are to be used. This is typically because of the relatively strong attraction that silica particles have for one another resulting in the need for very high shear or multiple mix passes (e.g. two or more mixing states) to obtain good dispersion of the silica in rubber.
Thus, in one aspect, is desired, for the practice of this invention, to provide a method of providing a relatively homogeneous dispersion of silica in rubber.
While the silica itself is not normally as good reinforcing agent as carbon black for rubber products, it is a desirable pigment for blending with rubber for the purpose of obtaining an increase in low strain dynamic stiffness without as much hysteresis as carbon black while maintaining tire treadwear performance.
The silica pigment typically has an average particle size in the range of about 100 to about 350 angstroms.
Such silicas are relatively well known to those having skill in the art of rubber compounding.
In some cases, relatively high levels (concentrations) of silica in rubber is desired, particularly where a stiff rubber compound is desired but low hysteresis of the rubber is also desired.